(1) Field of the Invention:
The present invention relates to a rubber composition suitable for a pneumatic tire which can concurrently satisfy criteria for rolling resistance, wet skid resistance, breakage resistance and wear resistance.
(2) Description of the Prior Art:
Recently, resource saving and energy saving have been socially demanded and investigations for developing so-called low fuel consumption tires have been carried out in order that the power loss is reduced.
It has been generally known that an automobile provided with a tire having a low rolling resistance is more efficient in the gasoline consumption, and the tire is accordingly called a so-called low fuel-consumption tire. In order to lower the rolling resistance, materials having a low glass transition temperature, such as cis-polybutadiene rubber and the like, or materials having a low hysteresis loss, such as natural rubber and the like, as a tread rubber are generally used.
However, these rubbers are extremely low in running stability, such as braking performance on wet road and wet driving performance, and it has been very difficult to concurrently satisfy the requirements for running stability on wet road and lowering the rolling resistance.
Recently, as described in Japanese Patent Laid-open Application No. 62,248/79, improvements in the rolling resistance and running stability are attained by using only styrene-butadiene copolymer (SBR) having styrene content of 20-40% by weight and 1,2-bond content in the bonded butadiene of 50-80% by weight. But all of the polymer consists of said SBR, so that satisfactory results have not been always obtained.
Thus, since all the polymer consists of said SBR, running stability is improved than the prior SBR but rolling resistance is not yet satisfactory. Moreover, in this prior art, conventional solution polymerization-type SBR obtained by using organolithium compound is used. Therefore, a rubber composition containing the SBR is poor in breakage strength and in elongation at break. Hence, there are various problems in running of the tire or in the production thereof. The former problem is the formation of cuts and cracks on the tread surface during running of the tire on a nonpaved road, and the latter problem is the breakage of the tread caused by the mold when a vulcanized tire is taken out from the mold, that is, so-called tread scratch formed by the mold. Furthermore, the above described SBR has a glass transition temperature higher than that of conventional SBR, and therefore the rubber composition containing the above described SBR is apt to be hardened. The tire tread often fails due to the low-temperature brittleness.
Accordingly, attaining a rubber composition concurrently satisfying both the rolling resistance and running stability while maintaining excellent breakage and wear resistances remains an unsolved problem.
The inventors have diligently studied a solution to the above described drawbacks. As described in U.S. Patent Application Ser. No. 231,529, the deformation at the friction surface of the tread occurs at high speed, and the hysteresis loss in the deformation at high speed has a great influence on the friction force, that is, the running stability of the tire. The deformation of the tread owing to ground contact, which has a great influence on rolling resistance of the tire, corresponds to the rotation speed of the tire, so that the hysteresis loss at deformations of less than 100 Hz has a great influence on rolling resistance. According to the temperature-time deducibility by Williams, Landel and Ferry, it is supposed that the hysteresis loss at high speed controlling running stability conforms to the hysteresis loss measured at a lower temperature than the temperature at which the tire is used. Hence, the inventors have studied the condition under which the hysteresis loss is increased in order to improve the running stability. They found that when the hysteresis loss measured at about 0.degree. C. in the dynamic measurement of 1.5 Hz is larger and the hysteresis loss at 50.degree.-70.degree. C. is smaller, rubber compositions wherein both running stability and low rolling resistance are concurrently improved, can be obtained.
Based on the above described discovery, the inventors have disclosed in U.S. Patent Application Ser. No. 231,529 that high vinyl SBR having a 1,2-bond content in butadiene unit of not less than 60% by weight can somewhat improve concurrently low rolling resistance and running stability. However, since such high vinyl SBR is insufficient in breakage resistance and wear resistance, the high vinyl SBR must be blended with other diene rubber. Moreover, unless a large amount of other diene rubber is blended, the resulting rubber can not be practically used. Consequently, when high vinyl SBR is used, the resulting rubber composition is satisfactory in the rolling resistance, but is poor in running stability.
The inventors have further disclosed in U.S. Patent Application Ser. No. 251,843 that a rubber composition consisting mainly of an amorphous gradient high vinyl butadiene-styrene copolymer rubber having an average content of 1,2-bond in butadiene unit being not less than 60% by weight, in which copolymer the 1,2-bond content uniformly varies along the molecular chain of the copolymer, can improve running stability and low rolling resistance of a tire while maintaining breakage strength of the tire.
However, running stability of a tire on wet road surface in a highway is an important property of the tire in view of safety running, and the development of low fuel consumption tires having high performance has been required from the view point of resource saving. With these problems in mind, the inventors have made various investigations with respect to styrene-butadiene copolymer rubber in order to further improve the breakage strength, low rolling resistance and running stability of tire. They have ascertained that the above described various properties can be improved by the use of a specifically limited rubber composition consisting mainly of a rubber containing at least 20 parts by weight, based on 100 parts by weight of the rubber, of a specifically limited high vinyl butadiene-styrene copolymer rubber, and accomplished the present invention.